The question is:
19.5.4
Show that the s-wave phase shift for a square well of depth V0 and range r0 is
δ0 = -k r0 + tan-1{(k/k') tan(k'r0)}
where k' and k are the wave numbers inside and outside the well. For k small, kr0 is some small number and we ignore it. Let us see what happens...